The Mermaid skirt is a wonderful pencil skirt with a frilly hem.
The skirt is most often knee-length, but can be longer. I have never seen anything longer than ankle-length, floor-length means you are in ball-gown territory.
The mermaid name is because of it's shape - hip hugging at first, to gently "fan" out at the hem, just like a Mermaids' fish-tail.
It is a mature choice for those days when a poofy Lolita skirt feels to over-dressed and this skirt is great for work or school.
From Fanny Rosies wardrobe: Victorian Maiden Rococo Bouquet
It is definitely not considered to be Lolita, but is sold by many Japanese Lolita brands and worn by many Classic Lolitas.
Coordinated in the right way, it may be considered Aristocrat (more dark and gothic) or Otome (more "girly" and light).
Some of the big Lolita brands that have released Mermaid Skirts are Victorian Maiden, Juliette et Justine, Metamorphose Temps de Fille and Atelier Boz.
Atelier Boz - Long Mermaid Skirts
Victorian Maiden "Elegant Check Skirt"